Optical transmission has become the darling of communication technology because of the enormous bandwidth that is available on an optical fiber. Such bandwidth enables thousands of telephone conversations and hundreds of television channels to be transmitted simultaneously over a hair-thin fiber that is generally made from a high-quality glass material. Transmission capacity over an optical fiber is increased in WDM systems wherein several channels are multiplexed onto a single fiber --each channel operating at a different wavelength. However, in WDM systems, nonlinear interactions between channels, such as 4-photon mixing, severely reduces system capacity. This problem has been largely solved by U.S. Pat. No. 5,327,516 (the '516 patent), which discloses an optical fiber that reduces these nonlinear interactions by introducing a small amount of chromatic dispersion at the operating wavelengths. It is noted that as the number of WDM channels to be transmitted over a single fiber increases, so too does the optical power carried by the optical fiber. And as the power increases, so too do the nonlinear effects. Accordingly, it is desirable for an optical fiber to provide a small amount of chromatic dispersion to each of the WDM channels.
Important advances have been made in the quality of the glass material (nearly pure silica--SiO.sub.2) used in making optical fibers. In 1970, an acceptable loss for glass fiber was in the range of 20 dB/km; whereas today, losses are generally below 0.25 dB/km. Indeed, the theoretical minimum loss for glass fiber is about 0.16 dB/km, and it occurs at a wavelength of about 1550 nanometers (nm). Nature appears to have smiled benignly upon optical transmission in this wavelength region because this is where Erbium-doped fiber amplifiers operate, and they have become the most practical optical amplifiers available. In such an amplifier, the Erbium ions, that the glass fiber has been doped with, are "pumped" with energy in a first wavelength region (e.g., 980 nm), and release that energy into a second wavelength region (e.g., 1530-1565 nm) when the Erbium ions are stimulated by transmitted optical signals in that second wavelength region. Such amplifiers are fundamental components in WDM systems where a broad spectrum of optical signals need to be amplified. Indeed, the transmission of one terabit per second (1 Th/s=1000 Gb/s) has already been demonstrated using twenty-five (25) adjacent channels, independent modulation of each of two polarization modes per channel, and other techniques. And while it is desirable to operate WDM systems in the 1530-1565 nm wavelength region (the Erbium amplifier region), present-day fiber designs undesirably have large differences in chromatic dispersion over the Erbium amplifier region.
Substantial effort has been devoted to the design of optical fibers having a flat dispersion characteristic across a broad wavelength region in order to accommodate transmission at both 1310 nm and 1550 nm. However, such "dispersion-flattened" fibers have achieved little or no commercial success because of excessive bending loss and tight manufacturing tolerances.
One optical fiber that provides a low-dispersion slope across the Erbium amplifier region has a refractive-index profile that resembles a donut, and it is shown at pages 259-260 of the OFC '95 Technical Digest in an article entitled: Dispersion-shifted single-mode fiber for high-bit-rate and multiwavelength systems. This design comprises a ring of high index material surrounding a core of low index material. However, the transmission loss associated with such a profile is in the order of 0.22 dB/km at 1550 nm, which is at least ten-percent (10%) higher than desirable. And while the disclosed design appears useful in providing negative chromatic dispersion with a low slope in the Erbium amplifier region, it does not appear to offer positive chromatic dispersion with a similarly low slope in the Erbium amplifier region.
Accordingly, what is desired, but does not appear to be disclosed in the prior art, is an optical fiber that is suitable for operation in the Erbium amplifier region having: (i) a transmission loss that is less than 0.22 dB/km at 1550 nm; (ii) a small amount of chromatic dispersion (i.e., an absolute magnitude of at least 0.8 ps/(nm-km)); and (iii) a chromatic dispersion characteristic having a low slope (i.e., less than 0.05 ps/(nm.sup.2 -km)).